XML : Altova Mapforce - checking input XML values including empty/Null and replacing with default value.

Altova Mapforce - checking input XML values including empty/Null and replacing with default values. Not able in XLST to check a element value and replace with a replacement vlaue if required.

For example the sending XML should contain a date as 00000000 and if that is the case, I need to substring the first 8 characters of another data element that is a date+time, that's the reason for the substring.

Then if the element is blank/empty/null I need to replace with a value to ensure the next process step will not fail due to missing key data.

Cheers